Low contact resistance PEM fuel cell

ABSTRACT

A PEM fuel cell having a current collector comprising a polymer composite and a diffusion media engaging said polymer composite. The polymer composite has a hyperconductive surface layer engaging the diffusion media to reduce the contact resistance therebetween. The hyperconductive surface layer is formed by depositing or smearing an electrically-conductive material on the surface of the polymer composite.

This is a divisional application of U.S. Ser. No. 09/997,190 filed Nov.20, 2001, now U.S. Pat. No. 6,811,918.

TECHNICAL FIELD

This invention relates to PEM fuel cells, and more particularly toelectrical current collectors (e.g. bipolar plates) therefor that aremade from polymer composite materials, and have low contact resistancewith adjacent diffusion media.

BACKGROUND OF THE INVENTION

Fuel cells have been proposed as a power source for electric vehiclesand other applications. One known fuel cell is the PEM (i.e., ProtonExchange Membrane) fuel cell that includes a so-called“membrane-electrode-assembly” (MEA) comprising a thin, solid polymermembrane-electrolyte having an anode on one face of themembrane-electrolyte and a cathode on the opposite face of themembrane-electrolyte. The anode and cathode typically comprise finelydivided carbon particles, very finely divided catalytic particlessupported on the carbon particles, and proton conductive materialintermingled with the catalytic and carbon particles. One suchmembrane-electrode-assembly and fuel cell is described in U.S. Pat. No.5,272,017 issued Dec. 21, 1993 and assigned to the assignee of thepresent invention. The membrane-electrode-assembly is sandwiched betweena pair of electrically conductive current collectors for the anode andcathode, which current collectors typically contain a number of landsthat define a plurality of channels or grooves for supplying the fuelcell's gaseous reactants (i.e., H₂ & O₂/air) to the surfaces of therespective anode and cathode.

Multi-cell PEM fuel cells comprise a plurality of the MEAs stackedtogether in electrical series and separated one from the next by agas-impermeable, electrically-conductive current collector known as abipolar plate. Such multi-cell fuel cells are known as fuel cell stacks.The bipolar plate has two working faces, one confronting the anode ofone cell and the other confronting the cathode on the next adjacent cellin the stack, and electrically conducts current between the adjacentcells. Current collectors at the ends of the stack contact only the endcells and are known as end plates.

A highly porous (i.e. ca. 60%-80%), electrically-conductive material(e.g. cloth, screen, paper, foam, etc.) known as “diffusion media” isinterposed between the current collectors and the MEA and serves (1) todistribute gaseous reactant over the entire face of the electrode,between and under the lands of the current collector, and (2) collectscurrent from the face of the electrode confronting a groove, and conveysit to the adjacent lands that define that groove. One known suchdiffusion media comprises a graphite paper having a porosity of about70% by volume, an uncompressed thickness of about 0.17 mm, and iscommercially available from the Toray Company under the name Toray 060.

In an H₂—O₂/air PEM fuel cell environment, the current collectors are inconstant contact with highly acidic solutions (pH 3-5) containing F⁻,SO₄ ⁻⁻, SO₃ ⁻, HSO₄ ⁻, CO₃ ⁻⁻, and HCO₃ ⁻, etc. Moreover, the cathodeoperates in a highly oxidizing environment, being polarized to a maximumof about +1 V (vs. the normal hydrogen electrode) while being exposed topressurized air. Finally, the anode is constantly exposed to hydrogen.Hence, the current collectors must be resistant to a hostile environmentin the fuel cell. Accordingly, current collectors have heretofore beeneither (1) machined from pieces of graphite, (2) molded from polymercomposite materials comprising about 70% to about 90% % by volumeelectrically-conductive filler (e.g. graphite particles or filaments)dispersed throughout a polymeric matrix (thermoplastic or thermoset), or(3) fabricated from metals coated with polymer composite materialscontaining about 30% to about 40% by volume conductive particles. Inthis later regard, see co pending United States patent Application,Fronk et al Ser. No. 09/456,478, filed Dec. 7, 1999 which (1) isassigned to the assignee of this invention, (2) is incorporated hereinby reference, and (3) discloses current collectors made from metalsheets coated with a corrosion-resistant, electrically-conductive layercomprising a plurality of electrically conductive, corrosion-proof (i.e.oxidation-resistant and-acid resistant) filler particles dispersedthroughout a matrix of an acid-resistant, water insoluble,oxidation-resistant polymer that binds the particles together and to thesurface of the metal sheet. Fronk et al-type composite coatings willpreferably have a resistivity no greater than about 50 ohm-cm and athickness between about 5 microns and 75 microns depending on thecomposition, resistivity and integrity of the coating. The thinnercoatings are preferred to achieve lower IR drop through the fuel cellstack.

Lightweight metals such as aluminum and their alloys have also beenproposed for use in making fuel cell current collectors. Unfortunately,such metals are susceptible to dissolution in the hostile PEM fuel cellenvironment. Accordingly, it has been proposed to coat lightweight metalcurrent collectors with a layer of metal or metal compound, which isboth electrically conductive and corrosion resistant to thereby protectthe underlying metal. See for example, Li et al RE 37,284E, issued Jul.17, 2001, which is assigned to the assignee of the present invention,and discloses a lightweight metal core, a stainless steel passivatinglayer atop the core, and a layer of titanium nitride (TiN) atop thestainless steel layer.

SUMMARY OF THE INVENTION

The present invention relates to PEM fuel cells using current collectorsmade either entirely, or at least in part, from polymer compositematerials such as described above. A high contact resistance existsbetween the diffusion media and current collectors made from suchcomposite materials owing to the fact that the diffusion media is porous(ca. 60%-80% porosity) and the surface of the composite materialengaging the diffusion media contains less than 70% (typically about 30%to about 40%) by volume of conductive filler (with the remainder beingnon-conductive polymer matrix material). Hence, current flow between thediffusion media and the composite material occurs only at sites where aconductive strand of the diffusion media contacts the conductive fillerin the composite material. No current flows at sites (1) where adiffusion media strand meets the polymer matrix material of thecomposite, or (2) where a composite's conductive filler meets a pore inthe diffusion media.

The present invention reduces the contact resistance between thediffusion media and composite-containing current collectors in PEM fuelcells. More specifically, the present invention is directed to a PEMfuel cell having at least one cell comprising (1) a pair of oppositepolarity electrodes each having a first face exposed to a fuel cellreactant and a second face engaging a membrane-electrolyte interjacentsaid electrodes, (2) a porous, electrically-conductive diffusion mediaengaging the first face for distributing reactant over, and conductingelectrical current from, the first face, and (3) a current collector(e.g. a bi-polar plate) engaging the diffusion media for conductingelectrical current from the media. The current collector comprises apolymer composite that has a first electrical conductivity and comprisescorrosion-proof electrically conductive filler dispersed throughout anoxidation-resistant and acid-resistant, water-insoluble polymericmatrix. The polymeric matrix may be either a thermoplastic or athermoset material, and will preferably be selected from the groupconsisting of epoxies, polyamide-imides, polyether-imides, polyphenols,fluro-elastomers, polyesters, phenoxy-phenolics, epoxide-phenolics,acrylics, and urethanes. The electrically conductive filler in thecomposite (1) are preferably selected from the group consisting of gold,platinum, graphite, conductive carbon, palladium, rhodium, ruthenium,and the rare earth metals, and (2) may take many physical forms (e.g.elongated filaments, or spheroidal, flake, fibrillose particles oraggregates of such particles Discrete fibrilose particles willpreferably be oriented generally in the direction current flows throughthe current collector, such as described in copending United Statespatent Application Blunk et al., U.S. Ser. No. 09/871,189 filed May 31,2001, which is assigned to the assignee of the present invention and isincorporated herein by reference. The invention is particularlyeffective with current collectors made from composites having fillerscomprising conductive filaments (e.g. graphite or carbon) extendingthrough the thickness of the current collector (i.e. in the direction ofcurrent flow through the collector).

The invention comprehends an oxidation-resistant and acid-resistantsurface layer that covers the composite component of the currentcollector and engages the diffusion media. The surface layer has asecond electrical conductivity that is greater (hereafterhyperconductive) than the conductivity of the underlying polymercomposite material, and may be a discrete layer adhering to thecomposite or an integral layer formed by embedding additional conductiveparticles in the surface of the composite, or smearing existing fillerfrom the composite over the exterior surface of the composite. Thehyperconductive surface preferably has a resistivity at least one orderof magnitude less than the underlying composite, and most preferably, atleast 100 times less than the underlying composite, and serves to reducethe contact resistance between the polymer composite material and thediffusion media by shunting electrical current passing through the mediato the conductive particles in the polymer composite that reside at theinterface between the surface layer and the composite. The invention isseen to be most beneficial at low (i.e. <150 psi) stack compressionpressures, but is also effective at high (i.e. >200 psi) stackcompression pressures, especially with polymer composites havingconductive particle loadings less than about 70% by volume.

According to one embodiment of the invention, the entire currentcollector is made (e.g. molded) from the polymer composite material andthen coated with the hyperconductive surface layer. In anotherembodiment, the current collector comprises a metal substrate (e.g. astamped metal sheet) that underlies a layer of polymer compositematerial, which, in turn, is coated, with the hyperconductive surfacelayer of the present invention. In still another embodiment, the metalsubstrate comprises a first acid-soluble metal (e.g. aluminum)underlying a second acid-insoluble, oxidizeable metal (e.g. titanium orstainless steel), a polymer layer atop the second metal, and thehyperconductive layer of the present invention atop the polymercomposite coating.

According to a preferred embodiment of the invention, thehyperconductive surface layer comprises a plurality of discrete,abutting, oxidation-resistant and acid-resistant,electrically-conductive particles (most preferably graphite) embedded ina surface of the composite so as to provide a higher concentration ofconductive particles at the surface than throughout the remainder of thecomposite. Suitable alternative particles include gold, platinum,conductive carbon, palladium, rhodium, ruthenium, and the rare earthmetals (i.e. the same particles as are used in the polymer composite).According to another embodiment, the hyperconductive surface layercomprises a continuous, oxidation-resistant, and acid-resistant,electrically-conductive film (e.g. metal, graphitic carbon,hyperconductive polymer composite, etc.) on the surface of thecomposite. The continuous film will preferably be vapor deposited,sprayed or electrolessly deposited onto the composite using conventionalPhysical Vapor Deposition (PVD), spraying or electroless (a.k.a.autocatalytic) deposition techniques well known in the art.

The present invention also comprehends a preferred process for making aPEM fuel cell current collector comprising the steps of forming thecurrent collector at least in part from a polymer composite materialcomprising a plurality of electrically conductive first particlesdispersed throughout an oxidation-resistant and acid-resistant,water-insoluble polymeric matrix, and adhering a sufficient quantity ofsecond electrically conductive particles to a surface of the compositematerial to provide that surface with a conductivity significantlygreater (i.e. orders of magnitude greater) than that of the underlyingcomposite material. The second particles may be applied to the compositeby spraying, brushing, sifting, fluidized bed immersion or the like, andmay be imbedded in the surface by impingement or simply stuck to thesurface while it is in a tacky state. According to a preferred method,the current collector is made by (1) coating an electrically conductivesubstrate (i.e. composite or metal) with a tacky layer of uncured orundried composite material comprising a plurality of electricallyconductive first particles dispersed throughout an oxidation-resistantand acid-resistant, water-insoluble polymer, (2) depositing a pluralityof electrically conductive second particles onto a surface of the tackylayer to increase the conductivity of the surface over the conductivityof the remainder of the composite material, and (3) curing/drying theuncured/undried coating material. Most preferably, the second particleswill be sprayed (ala sand blasting) onto the composite material withsufficient pressure to embed the particles in the uncured/undriedcoating material. The coating is then cured/dried to anchor theparticles in place. Following curing/drying, any unbonded particles arebrushed or blown from the surface. Alternatively, the surface of thepolymer composite may either be heated or wetted with a solvent prior tospraying to soften the surface and render it more receptive to embeddingthe second particles. According to still another alternative, thesurface of a dried/cured polymer composite is gently abraded (e.g. withfine sand paper) to remove any polymer skin that may have formed overthe conductive filler and to smear the abraded filler over theunderlying surface so as to increase its conductivity over that of thebulk of the composite material underlying the surface.

The present invention reduces the contact resistance between acomposite-containing current collector and the diffusion media, which,in turn, permits the making of PEM fuel cells which require less stackcompression, are smaller, are more efficient and have lower heat loads.Reducing compression alone improves stack durability, permits the use ofthinner side and end plates, improves gas flow under the lands of theflow field, and provides more uniform current distribution.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will better be understood when considered in the light ofthe following detailed description of certain specific embodimentsthereof which is given hereafter in conjunction with the several figuresin which:

FIG. 1 is a schematic, exploded, isometric, illustration of aliquid-cooled PEM fuel cell stack (only two cells shown);

FIG. 2 is an exploded, isometric view of a bipolar plate useful with PEMfuel cell stacks like that illustrated in FIG. 1;

FIG. 3 is a sectioned view in the direction 3-3 of FIG. 2;

FIG. 4 is a magnified portion of the bipolar plate of FIG. 3;

FIG. 5 is a magnified sectioned view of a bi-polar plate depictinganother embodiment of the present invention;

FIG. 6 is a magnified portion of FIG. 4;

FIG. 7 is a magnified sectional view of a bipolar plate depictinganother embodiment of the present invention;

FIG. 8 is a magnified sectional view of a PEM half-cell illustrating howthe invention operates; and

FIGS. 9 and 10 are graphs comparing the contact resistances forcomposite-coated stainless steel and titanium plates with and withoutthe more conductive surface layer of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 depicts a two cell, bipolar PEM fuel cell stack having a pair ofmembrane-electrode-assemblies (MEAs) 4 and 6 separated from each otherby an electrically conductive, liquid-cooled, bipolar plate 8. The MEAs4 and 6, and bipolar plate 8, are stacked together between stainlesssteel clamping plates 10 and 12, and current collector end plates 14 and16. The clamping plates 10 and 12 apply compressive force to the stackby means of bolts (not shown) that pass through openings 13 at thecorners of the clamping plates 10, 12. The end plates 14 and 16, as wellas both working faces of the bipolar plate 8, contain a plurality ofgrooves or channels 18, 20, 22, and 24 for distributing fuel and oxidantgases (i.e., H₂ & O₂) to the MEAs 4 and 6. Nonconductive gaskets 26, 28,30, and 32 provide seals and electrical insulation between the severalcomponents of the fuel cell stack. Gas permeable carbon/graphitediffusion media 34, 36, 38 and 40 press up against the electrode facesof the MEAs 4 and 6. The end plates 14 and 16 press up against thecarbon/graphite diffusion media 34 and 40 respectively, while thebipolar plate 8 presses up against the carbon/graphite media 36 on theanode face of MEA 4, and against carbon/graphite media 38 on the cathodeface of MEA 6. Oxygen is supplied to the cathode side of the fuel cellstack from storage tank 46 via appropriate supply plumbing 42, whilehydrogen is supplied to the anode side of the fuel cell from storagetank 48, via appropriate supply plumbing 44. Alternatively, air may besupplied to the cathode side from the ambient, and hydrogen to the anodefrom a methanol or gasoline reformer, or the like. Exhaust plumbing (notshown) for both the H₂ and O₂/air sides of the MEAs will also beprovided. Additional plumbing 50, 52 and 54 is provided for supplyingliquid coolant to the bipolar plate 8 and end plates 14 and 16.Appropriate plumbing for exhausting coolant from the plate 8 and endplates 14 and 16 is also provided, but not shown.

FIG. 2 is an isometric, exploded view of a bipolar plate 56 comprising afirst exterior metal sheet 58, a second exterior metal sheet 60, and aninterior spacer metal sheet 62 interjacent the first metal sheet 58 andthe second metal sheet 60. The exterior metal sheets 58 and 60 are madeas thin as possible (e.g., about 0.002-0.02 inches thick), may be formedby stamping, by photo etching (i.e., through a photolithographic mask)or any other conventional process for shaping sheet metal. The externalsheet 58 has a first working face 59 on the outside thereof whichconfronts a membrane-electrode-assembly (not shown) and is formed so asto provide a plurality of lands 64 which define therebetween a pluralityof grooves 66 known as a “flow field” through which the fuel cell'sreactant gases (i.e., H₂ or O₂) flow in a tortuous path from one side 68of the bipolar plate to the other side 70 thereof. When the fuel cell isfully assembled, the lands 64 press against the carbon/graphite media 36or 38 (see FIG. 1), which, in turn, press against the MEAs 4, and 6respectively. For drafting simplicity, FIG. 2 depicts only two arrays oflands and grooves. In reality, the lands and grooves will cover theentire external faces of the metal sheets 58 and 60 that engage thecarbon/graphite diffusion media 36 and 38. The reactant gas is suppliedto grooves 66 from a header or manifold groove 72 that lies along oneside 68 of the fuel cell, and exits the grooves 66 via anotherheader/manifold groove 74 that lies adjacent the opposite side 70 of thefuel cell. As best shown in FIG. 3, the underside of the sheet 58includes a plurality of ridges 76 which define therebetween a pluralityof channels 78 through which coolant passes during the operation of thefuel cell. As shown in FIG. 3, a coolant channel 78 underlies each land64 while a reactant gas groove 66 underlies each ridge 76.Alternatively, the sheet 58 could be flat and the flow field formed in aseparate sheet of material.

Metal sheet 60 is similar to sheet 58. The internal face 61 (i.e.,coolant side) of sheet 60 is shown in FIG. 2. In this regard, there isdepicted a plurality of ridges 80 defining therebetween a plurality ofchannels 82 through which coolant flows from one side 69 of the bipolarplate to the other 71. Like sheet 58 and as best shown in FIG. 3, theexternal side of the sheet 60 has a working face 63 having a pluralityof lands 84 thereon defining a plurality of grooves 86 through which thereactant gases pass. An interior metal spacer sheet 62 is positionedinterjacent the exterior sheets 58 and 60 and includes a plurality ofapertures 88 therein to permit coolant to flow between the channels 82in sheet 60 and the channels 78 in the sheet 58 thereby breaking laminarboundary layers and affording turbulence which enhances heat exchangewith the inside faces 90 and 92 of the exterior sheets 58 and 60respectively.

FIG. 4 is a magnified view of a portion of FIG. 3 and shows the ridges76 on the first sheet 58, and the ridges 80 on the second sheet 60bonded (e.g. by brazement 85) to the spacer sheet 62. The working faces59 and 63 of the bipolar plate are covered with a coating of a compositematerial comprising an electrically-conductive, oxidation resistant, andacid-resistant protective material 94 having a resistivity less thanabout 50 ohm-cm, and comprising a plurality of oxidation-resistant,acid-insoluble, conductive particles (i.e. less than about 50 microns)dispersed throughout an acid-resistant, oxidation-resistant polymermatrix. The conductive filler particles are selected from the groupconsisting of gold, platinum, graphite, carbon, palladium, niobium,rhodium, ruthenium, and the rare earth metals. Most preferably, theparticles will comprise conductive carbon and graphite at a loading ofabout 25% by weight. The polymer matrix comprises any water-insolublepolymer that can be formed into a thin adherent film and that canwithstand the hostile oxidative and acidic environment of the fuel cell.Hence, such polymers, as epoxies, polyamide-imides, polyether-imides,polyphenols, fluro-elastomers (e.g., polyvinylidene flouride),polyesters, phenoxy-phenolics, epoxide-phenolics, acrylics, andurethanes, inter alia are seen to be useful with the composite coating.Cross-linked polymers are preferred for producing impermeable coatings,with polyamide-imide thermosetting polymers being most preferred. Toapply the polymer composite layer, the polyamide-imide is dissolved in asolvent comprising a mixture of N-methylpyrrolidone, propylene glycoland methyl ether acetate, and about 21% to about 23% by weight of amixture of graphite and carbon black particles added thereto. Thegraphite particles range in size from about 5 microns to about 20microns and the carbon black particles range in size from about 0.5micron to about 1.5 microns. The mix is sprayed on to the substrate,dried (i.e. solvent vaporized), and cured to provide 15-30 micron thickcoating (preferably about 17 microns) having a carbon-graphite contentof about 38% by weight. It may be cured slowly at low temperatures (i.e.<400° F.), or more quickly in a two step process wherein the solvent isfirst removed by heating for ten minutes at about 300° F.-350° F. (i.e.,dried) followed by higher temperature heating (500° F.-750° F.) forvarious times ranging from about ½ min to about 15 min (depending on thetemperature used) to cure the polymer. As described hereinafter, thehyperconductive surface layer of the preferred embodiment of theinvention is applied before drying and curing while the composite isstill tacky.

The substrate metal 58, 60 forming the structural component of thecurrent collector comprises a corrosion-susceptible metal such as (1)aluminum which is dissolvable by the acids formed in the cell, or (2)titanium or stainless steel which are oxidized/passivated by theformation of oxide layers on their surfaces. The conductive polymercoating may be applied directly to the substrate metal and allowed todry/cure thereon, or the substrate metal (e.g., Al) may first be coveredwith an oxidizable metal (e.g., stainless steel) before the electricallyconductive polymer composite layer 94 is applied (see Li et al. supra).The composite layer may be applied in a variety of ways, e.g., brushing,spraying, spreading, or laminating a preformed film onto the substrate.

As shown in FIG. 5, the electrically-conductive polymer compositecoating 94 is applied to an acid-dissolvable substrate metal (e.g., Al)58, 60 which had previously been coated with a layer ofoxidizable/passivating metal 96. In this regard, a barrier/protectivelayer 96 of a metal (e.g. Ni/Cr-rich stainless steel) that forms a lowresistance, passivating oxide film is first deposited onto the substrate58, 60, and the barrier layer 96 then covered with the polymer compositelayer 94.

In accordance with the present invention, the composite component of thecurrent collector is provided with a hyperconductive outer surface layerthat has a significantly higher electrical conductivity than theremainder of the composite material underlying the surface layer. Hencefor example as shown in FIGS. 4 and 6, the bipolar plate comprises metal(e.g. stainless steel or titanium) plates 58 and 60, each coated with acomposite material 94 (ala Fronk et al. supra) which, in turn, has asurface layer 100, 102 thereon that has a higher electrical conductivitythan the remainder of the composite material underlying the outer layer.The surface layer 100, 102 engages the diffusion media and results in alower contact resistance than would otherwise exist between thecomposite layer 94 and the diffusion media absent the more conductivesurface layer. Similarly as shown in FIG. 5, corrosion-sensitive metalplates (e.g. aluminum) are first coated with a corrosion-resistant metallayer 96 (e.g. stainless steel, titanium, etc.), which, in turn, iscoated with a composite layer 94 followed by the more conductive layer100, 102 of the present invention.

FIG. 7 depicts another embodiment of the present invention wherein thebody of the current collector is made (e.g. molded) entirely ofcomposite material 104, and the more conductive surface layer 106 formedon the exterior surface of the composite that engages the diffusionmedia.

FIG. 8 depicts how the invention works and shows a half-cell of a PEMfuel cell having a current collector like that shown in FIGS. 4 and 6.More specifically, FIG. 8 depicts an enlarged portion of a PEM fuel cellaccording to the present invention including a current collectorcomprising a metal plate 108, an electrically conductive compositecoating 110 atop the metal plate 108, a hyperconductive surface layer112 on the composite coating 110 which is more electrically conductivethan the underlying composite coating 110, an electrically-conduciveporous diffusion media 114 (e.g. graphite paper) engaging thehyperconductive surface layer 112 and an MEA 116 engaging the diffusionmedia 114. The composite layer 110 comprises a plurality of conductiveparticles (e.g. graphite and carbon black) 111 dispersed throughout apolymeric matrix 113. A plurality of electrically conductive bridges 115are formed by abutting particle s 111, which bridges serve to conductcurrent flow through the composite 110. Alternatively, the bridges 115could comprise single filaments (e.g. graphite fibers). The MEA 116comprises a membrane-electrolyte 118, and a layer of catalyst 120forming an electrode on the surface of the MEA 116. In operation,electrons pass from the electrode 120 into the diffusion media 114 andthence to the hyperconductive layer 112. The electrons readily movelaterally through the hyperconductive layer 112 until a conductivebridge 115 is encountered that allows current to flow through thecomposite 110. Hence, the hyperconductive layer 112 essentially shuntsthe current from the more resistive areas of the interface 122 betweenthe composite 110 and the hyperconductive layer 112 to the moreconductive areas of that interface (i.e. where conductive particles 111reside).

The second particles of the hyperconductive layer of the presentinvention may be applied to the composite in a number of ways. Accordingto a preferred method, the composite material is sprayed onto aconductive substrate (i.e. metal or all polymer composite) as a mixtureof first conductive particles (e.g. graphite), polymer and solvent forthe polymer. Spraying leaves a tacky composite coating on the substrate.Dry second conductive particles (e.g. graphite) are then sprayed orotherwise applied onto the tacky composite coating and stick thereto. Atlow (i.e. <about 10 psi) spray pressures, the second conductiveparticles simply stick to the surface of the composite, whereas athigher (>about 40 psi) spray pressures the second particles become moredeeply embedded in the surface of the composite. Following deposition ofthe second particles, the composite is dried and/or cured by heating.Curing/drying time and temperature will vary with the composition of thepolymer matrix. Alternatively, the second particles may be applied tothe composite by brushing, sifting, fluidized bed or similar techniques.In the case of metal substrates, it may be desirable to deposit at leasttwo layers of composite before applying the more-conductive top layer inorder to provide extra corrosion protection for the metal substrate.When two composite layers are used, the first layer may be sprayed onand dried as described above. It may or may not be cured before applyingthe second composite layer. The hyperconductive layer of the presentinvention is applied to the second composite layer in the same manner asdescribed above. When the current collector is made (e.g molded)entirely from composite material, the second conductive particles of thehyperconductive layer of the present invention may be applied directlyto the surface of the composite material, or the surface of thecomposite material may be coated with a separate composite layer asdescribed above. In the former case, it is desirable to soften thesurface of the composite to make it more receptive to receiving andretaining the second conductive particles. Softening may be effected bywiping/spraying the surface with a suitable solvent, or by heating thesurface. The second conductive particles are applied to the compositesurface while it is still soft, followed by drying or cooling, asneeded, to anchor the second particles in place. Following curing,drying or cooling, which ever is used, loose unbonded second conductiveparticles are blown or brushed from the surface.

Mildly abrading the surface of a dried/cured composite material willalso make the surface of the composite more conductive (i.e.hyperconductive) than the underlying composite. Hence, an alternativetechnique for making a current collector according to the presentinvention is to make it at least in part from a composite material, andthen lightly sanding it with a fine abrasive to remove any polymer skinthat might be covering the filler at the surface, and to smear thefiller in the abraded layer over the underlying surface and therebyenhance its conductivity. For example with composites comprising about30% of a mixture of graphite and carbon black, lightly sanding with 0000grit SiC sand paper until the surface color changes from black to greyhas been found to be highly effective in rendering the surfacehyperconductive relative to the underlying polymer composite material

The hyperconductive surface layer of the present invention mayalternatively comprise a thin continuous film of conductive materialdeposited on the surface of the composite. By continuous is meant a filmthat can be peeled off as a unit as distinguished from a plethora ofdiscrete particles that touch each other, but are not integral with eachother. The films may have the same chemical composition as the secondparticles identified above, but are plated onto the composite by wellknown electroplating, electroless plating, PVD (physical vapordeposition) or sputtering techniques. In this regard, gold, ruthenium,palladium, rhodium and platinum can be readily electroplated onto thecomposite surface, and gold, palladium and platinum readilyelectrolessly plated on to the composite surface. PVD or sputtering canbe used to deposit all of the conductive materials identified above ascomprising the hyperconductive surface layer. In the electroplatingprocess, the composite is made the cathode in an electroplating cellhaving an electrolyte containing a salt of the metal being deposited.When electrical current passes through the cell at an appropriatepotential, the desired metal deposits out on the surface of the cathodiccomposite. In the electroless (i.e. autocatalytic) process the surfaceof the composite is seeded with a catalyst and then exposed to a bathcontaining ions of the metal to be deposited. The catalyst initiatesreduction of the metal ions to elemental metal, which forms a metal filmon the surface of the substrate. In the PVD and sputtering processes,the material to be plated condenses on the substrate from a vaporthereof. The continuous hyperconductive film may also comprise a layerof hyperconductive polymer composite material having a higher fillerloading (e.g. >90%) than the underlying composite substrate. Such highlyloaded polymer composites may be rolled, brushed, doctor-bladed, orsprayed onto the substrate.

FIGS. 9 and 10 are graphs showing the reduced contact resistancesachievable by the present invention with stainless steel and titaniumsubstrates respectively. The titanium samples were polished with anabrasive pad to remove any insulating oxides thereon. The stainlesssteel samples were cathodically cleaned at 5-50 mA/cm² for 5 to 30minutes in a 0.1 to 1.0 molar sulfuric acid solution to reduce the oxidethickness. The thusly polished/cleaned samples were spray coated with afirst layer of composite material comprising about 30% by volumegraphite and carbon black particles dispersed throughout apolyamide-imide polymer matrix. The coated samples were flash dried at150° C. for 10 minutes to remove the solvent and solidify the coating.Without curing, the sample was coated a second time with the samecomposite material followed by spraying a hyperconductive layer ofgraphite flakes atop the second composite coating. The samples were thendried by flashing the solvent off at 150° C. for 10 minutes followed bycuring the first and second composite coatings at 260° C. for 15minutes. Loose, unbonded graphite particles were blown from the surface.The samples were then mated to diffusion media provided by the TorayCompany and identified as TGP-H-1.0T, and subjected to contactresistance tests. Control samples were made from the same stainlesssteel and titanium metals and coated with the same two layers ofcomposite materials, but were not provided with a hyperconductivesurface layer of graphite flakes. The results of those comparative testsare shown in FIGS. 9 (stainless steel) and 10 (titanium). Both figuresshow that the contact resistances for five samples prepared according tothe present invention (i.e. with the hyperconductive layer) weresignificantly lower than the contact resistances for five controlsamples over a wide range pressures applied to the samples and diffusionmedia. Other tests demonstrated the stability of the hyperconductivelayer in that the contact resistance of samples made in accordance withthe present invention, as set forth above, did not increase duringcorrosion testing in a fuel cell-like environment.

While the invention has been described in terms of specific embodimentsthereof it is not intended to be limited thereto but rather only to theextent set forth hereafter in the claims, which follow.

1. A method comprising making a current collector for a fuel cellcomprising coating an electrically conductive substrate with a tackylayer of uncured or undried material comprising a corrosion-proof,electrically-conductive filler dispersed throughout anoxidation-resistant and acid-resistant polymer, thereafter embedding aplurality of electrically-conductive particles in a surface of saidlayer so as to increase the conductivity of said surface over theconductivity of the remainder of said material, and thereafter curing ordrying said layer.
 2. A method according to claim 1 wherein theembedding comprising spraying said particles onto said surface at apressure greater than 40 psi.
 3. A method according to claim 1 furthercomprising molding said electrically conductive substrate from acomposite material comprising corrosion-proof, electricallyconductivefiller dispersed throughout an oxidation-resistant and acid-resistant,water-insoluble polymer.
 4. A method according to claim 1 wherein saidsubstrate comprises a metal.
 5. A method as set forth in claim 1 whereinthe particles comprise at least one of gold, platinum, palladium,rhodium, ruthenium, or rare earth metals.
 6. A method as set forth inclaim 1 wherein the particles comprise conductive carbon.
 7. A method asset forth in claim 1 wherein the particles are present in a higherconcentration at the surface than the remainder of the composite.
 8. Amethod as set forth in claim 1 wherein the embedding comprises sprayingsaid particles onto the surface of the layer at a pressure greater than40 psi.
 9. A method as set forth in claim 1 further comprising placing adiffusion media adjacent the collector so that the contact resistancebetween the diffusion media and collector is reduced by the increasedconductivity of the surface.
 10. A method as set forth in claim 9further comprising placing a membrane electrode assembly adjacent thediffusion media.